This invention relates to an electronic controlled fluid suspension system for controlling vehicle attitude.
In general, when a vehicle is started and accelerated, squat of the vehicle body occurs, when decelerated and stopped, on the other hand, dive of the body occurs. To cope with such phenomena and to improve ride comfort and maneuverability of the vehicle, various types of electronic controlled suspension systems for controlling inclination of the vehicle body due to the squat and the dive of the body have been developed. One of those suspension systems has been disclosed in Japan published unexamined utility model application No. 60-119631. In this prior art, when a braking deceleration exceeds a predetermined value G1, control valves are opened for a predetermined time interval to maintain a horizontal vehicle attitude. When the braking deceleration becomes lower than another predetermined value G2, another set of control valves are opened for the aforementioned time interval so as to execute the so-called return control, i.e., a reversing attitude control. Another type of electronic controlled suspension system has been disclosed in Japan published unexamined patent application No. 61-64512, in which characteristic of the suspension is changed in several stages according to the detected acceleration.
These prior art systems, however, still include some problems in the maintenance of ride comfort and maneuverability of the vehicle. For example, the prior art suspension systems cannot effect appropriate control corresponding to various braking operations, e.g., light-touch braking, deep braking or sudden braking. When the acceleration (deceleration) exceeds a predetermined level, the same prescribed control is carried out irrespective of braking conditions so that ride comfort and maneuverability controls are sometimes insufficient. On the other hand, when the suspension characteristic is determined according to the amount of acceleration, the control system is apt to respond to even a momentary change in the acceleration. As a consequence, execution of accurate control considering every changing manner of the acceleration and deceleration may be difficult, and accordingly ride comfort and maneuverability of the vehicle may be hard to realize.
The conventional return control also involves some shortcomings. When the return control is executed by following the process opposite to the vehicle attitude control, the control time interval t is determined by an estimation so that the vehicle attitude is not necessarily returned to the original state with accuracy. On the other hand, when the return control is executed by communicating air springs of front and rear wheels, much time is required for restoring the vehicle attitude since the speed of the air flow between the air springs becomes slower as the pressure difference between the air springs becomes smaller.